You're seeing just the function verify_ack_against_message, go back to MLLP.Ack module for more information.
Link to this function

verify_ack_against_message(bad_message, bad_ack)


  message :: String.t() | HL7.Message.t(),
  ack_message :: String.t() | HL7.Message.t()
) :: ack_verification_result()

Verifies the ack code in the ack message against the original HL7 message. Possible valid codes and their values include: AA -- Application Accept CA -- Application Accept AR -- Application Reject CR -- Application Reject AE -- Application Error CE -- Application Error

All other possible codes would be considered bad ack codes and the method returns :error